Annotate WebSockets.RegisterPrefixes with [Obsolete]

[alnikola]

Fixes#50530

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Issue Details

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Fixes #50530

Author:	alnikola
Assignees:	-
Labels:	area-System.Net
Milestone:	-

[stephentoub]

Did we discuss this one in API review?

Does it need a unique diagnostic ID or we're ok just with the general ID? (In this particular case I personally think the general ID is fine and it doesn't need to be assigned a unique one.)

The ref needs to be updated accordingly as well.

[alnikola]

@stephentoub Sorry, I didn't know that an API review is required for this since the issue #50530 explicitly asks to do it and a similar thing was done for QUIC API #49261.
Should I close this PR then?

[stephentoub]

Should I close this PR then?

No, this one's pretty simple. @bartonjs, @terrajobst, any concerns? This API is useless and is already hidden.

[bartonjs]

any concerns?

Not from me. I like Obsoleting things 😄.

It'd be a good idea to fix the tests so they compile, though 😏

[alnikola]

Build fixed.